Depth And Three-Dimensional Trajectory Tracking Control Of Autonomous Underwater Vehicles In Complex Marine Environment

Due to the enormous undersea technological improvements obtained in the last decades,autonomous underwater vehicles(AUVs)have been increasingly used to perform the risky ocean missions such as offshore oil and gas exploration and exploitation,submarine pipelines inspection,oceanographic mapping,deep sea archaeology and obviating torpedoes,etc.During the execution of the above missions,it usually requires that the AUVs should have the ability to well regulate the depth and to precisely track the desired trajectory.However,the dynamic model of the vehicle has multivariable,highly nonlinear,strongly coupled and uncertain properties,coupled with the time-varying external disturbances which are hard to measure in the ocean environment,making the trajectory tracking control of AUVs a challenging task.In addition,the trajectory tracking control of AUVs demands to force the vehicle to reach and follow a time-varying parameterised trajectory,which poses strong constraint on convergent time,and consequently increasing the difficulty in the controller design.In order to overcome the aforementioned challenges and promote the development of the depth and trajectory tracking control technology of AUVs,this dissertation investigates the depth and three-dimensional trajectory tracking control problem of AUVs in the presence of dynamic uncertainties and time-varying external disturbances,and provides some novel results for the depth and trajectory tracking control methods design of AUVs.The main results of the dissertation include:1.For the problem that the previous PID depth controllers for AUVs cannot accurately compensate for the output disturbances,model uncertainties and input time delay,and the previous H? and H2 depth controllers design for AUVs do not specifically consider the input time delay,a robust H2 optimal depth control method is proposed.The developed controller can effectively deal with the input time delay,output disturbances as well as model uncertainties of the vehicle,and can quantitatively tune the nominal performance and robustness of the system.Comparative simulations with the existing pitch-and-depth loop PD depth controller demonstrate that the proposed controller provides higher tracking accuracy,better output disturbances rejection ability,stronger robustness against model uncertainties,and smaller fin angle input.However,the proposed controller requires the transfer functions of the AUVs systems.2.For the problem that the previous control methods for the trajectory tracking control of AUVs such as adaptive control,backstepping control,neural network control,fuzzy control and model predictive control can only guarantee the robust stability of the trajectory tracking,but cannot guarantee the transient responses during tracking,three exponentially convergent robust controllers,namely,the min-max type controller,the saturation type controller and the smooth transition type controller are proposed.The exponentially convergent analytic expressions of all the tracking errors are derived,which illustrate how to get the desired transient responses of the tracking errors by adjusting the controller parameters.Comparative simulations with the existing RISE-based controller of AUVs show that the three proposed controllers cannot only provide faster convergence speed but also can compensate for the non-smooth disturbances(e.g.random disturbances)which can typically arise in the subaquatic environment.However,these three controllers need upper bounds of the uncertainties and disturbances and do not consider the inertial uncertainties in the vehicle dynamics.3.For the problem that the three controllers proposed in main result 2 need upper bounds of the uncertainties and disturbances and do not consider the inertial uncertainties in the vehicle dynamics,as well as the previous global finite-time stable tracking controller and adaptive nonsingular terminal sliding mode controller for AUVs can only guarantee the boundedness of the tracking error and thus the tracking accuracy needs to be improved,two finite-time stable tracking controllers,namely,double-loop adaptive integral terminal sliding mode control scheme and double-loop adaptive fast integral terminal sliding mode control scheme are proposed.The two proposed controllers guarantee position and velocity tracking errors of the vehicle to locally converge to zero in finite time.Meanwhile,the bound information of the dynamic uncertainties(including the inertial uncertainties)and time-varying external disturbances is not required.Comparative simulations with the conventional double-loop adaptive integral sliding mode control scheme demonstrate that the two proposed control schemes offer faster convergence rate and stronger robustness.However,in contrast to the three controllers proposed in main result 2,these two controllers can only guarantee the transient responses of the tracking errors on the sliding surfaces,and cannot guarantee the transient responses of the tracking errors before they reach the sliding surfaces.4.For the problem that the two control schemes presented in main result 3 suffer from singularity,an adaptive nonsingular integral terminal sliding mode control(ANITSMC)scheme is proposed.The ANITSMC is first investigated and proposed for a general first-order uncertain nonlinear dynamic system.It does not have any singularity problem,does not need the bound information of the lumped system uncertainty,and meanwhile guarantees global finite-time convergence of the tracking error to zero.The proposed ANITSMC is then applied to the three-dimensional trajectory tracking control of AUVs.It overcomes the singularity problem existing in the two controllers presented in main result 3,and meanwhile ensures robust and fast trajectory tracking.Finally,comparative simulations with the conventional adaptive proportional-integral sliding mode control scheme show that the proposed control scheme offers faster convergence rate and better robustness.However,compared with the two control schemes presented in main result 3,the proposed control scheme losses the convergence rate,that is,it can only guarantee that the velocity tracking errors locally converges to zero in finite time,and the position tracking errors is no longer guaranteed to converge to zero in finite time,but to locally converge to zero exponentially.5.For the problem that the control scheme presented in main result 4 has slow convergence rate in the region far from the equilibrium point,an adaptive fast nonsingular integral terminal sliding mode control(AFNITSMC)scheme is proposed.The AFNITSMC is also first investigated and proposed for a general first-order uncertain nonlinear dynamic system.It guarantees fast,finite-time convergence both at a distance far from and at a close range of the equilibrium point.The proposed AFNITSMC is then applied to the three-dimensional trajectory tracking control of AUVs.Comparative simulations with the control scheme presented in main result 4 demonstrate that the proposed control scheme improves the convergence rate.Nevertheless,the structure of the proposed control scheme is more complicated than that of the control scheme presented in main result 4.6.For the problem that the previous chattering reduction methods used in the terminal sliding mode control of AUVs lose the tracking accuracy,and the existing adaptive second-order nonsingular terminal sliding mode control(ASONTSMC)scheme has slow convergence rate in the region far from the equilibrium point,an adaptive second-order fast nonsingular terminal sliding mode control(ASOFNTSMC)scheme is proposed.The proposed controller can eliminate the chattering without reducing the tracking precision,and meanwhile can ensure fast convergence in the region far from and close to the equilibrium point.Furthermore,the proposed controller does not require the bound information of the system uncertainty.Comparative simulations show that the proposed control scheme offers faster convergence rate than the existing ASONTSMC scheme,and verify the superiority of the proposed control scheme over the previous chattering reduction methods used in the terminal sliding mode control of AUVs.However,in comparison with the control scheme presented in main result 5,the proposed control scheme requires the acceleration measurement information.